You can perform an update into using a select, but I think he's talking about modifying the data coming from the select statement artibitrarily. In that case you have to create a new statement to the database an execute an UPDATE sql command separately from your SELECT query. That's the cleaner/better way to do it anyway.

For example, you may need to make changes to a dozen records, rather than 12 separate update calls, mark the records during the select you need to change and then update them all in a single update statement after all the records have been read.